Hydrogen sulfide is a toxic and corrosive gas. It can diffuse into downhole fluids, such as hydrocarbon fluids, produced from earth formations. Hydrogen sulfide corrodes infrastructure, reduces product value, and compromises safety and environmental compliance. Hence it is desirable to be able to detect the presence of hydrogen sulfide in the downhole fluids during early stages of producing reservoir assessment to determine suitable materials for components exposed to downhole fluids produced from wells drilled into such reservoirs. Detection of hydrogen sulfide in downhole fluids can also guide preventive treatments during production.
ASTM D5705 and UOP163 methods can be used for determining hydrogen sulfide levels. The ASTM D5705 method is based on the agitation of the sample and measurement of the gas concentration of hydrogen sulfide. Thus the method does not measure the hydrogen sulfide present in the liquid phase. The UOP 163 method is based on titration measurement of the liquid phase and relies on agitation of the sample under test at atmospheric pressure. A disadvantage of this test is that the method can lead to significant loss of entrained hydrogen sulfide, which may inaccurately represent an actual concentration of hydrogen sulfide in the liquid.
Chemiresistive sensors have been the subject of extensive research in the recent years. Conventional chemiresistive sensors are generally formulated and configured to sense a presence of an analyte in the vapor phase. However, to operate properly, such sensors require that the sensor be immersed in a vapor phase such that the only electrically conductive path of the sensor is through the sensing element. The electrical resistance of vapors is substantial, preventing any charge transfer through the vapor phase and ensuring that substantially all of the change transfer is through the sensing element of the chemiresistive sensor. Accordingly, such prior art chemiresistive sensors are not suitable for sensing an analyte present within a fluid (e.g., a liquid) of interest.